1. Field of the Invention
The present invention relates to the recovery of plutonium metal. In particular, the present invention relates to a process for dissolving plutonium metal in a sulfamic acid--fluoride solution. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Plutonium can be used as a reactor fuel, as a power source in satellites, and in a number of defense-related applications. In many of these applications, accumulation of fission products over a period of time necessitates withdrawal of the remaining plutonium for reprocessing and recovery of the plutonium. Fission products absorb neutrons that might otherwise be absorbed by fissionable plutonium.
Plutonium recovery and purification typically begins by dissolving plutonium-containing alloys or compounds in an acid solution. It is known to recover actinides such as plutonium from precipitates or slag by dissolution in nitric acid, as in the processes described by Knox, et al. (U.S. Pat. No. 2,938,768) and Hopkins (U.S. Pat. No. 2,898,186).
Plutonium and plutonium oxide are difficult to dissolve, even in highly concentrated acid solutions. Therefore, many processes depend on catalysts. Mills, et al. (U.S. Pat. No. 4,333,912) increase the solubility of plutonium dioxide and plutonium/uranium oxide fuels in nitric acid by adjusting the plutonium:uranium ratio so that the plutonium acts as an autocatalyst. Schulz (U.S. Pat. No. 3,222,125) first immerses an aluminum-based nuclear fuel in nitric acid containing mercuric ion catalyst, then increases the nitric acid concentration to complete dissolution. Schulz (U.S. Pat. No. 2,897,047) accelerates dissolution of metallic uranium by adding ortho-phosphoric acid to nitric acid.
Fluoride ions are used as catalysts for speeding up the dissolution of plutonium and plutonium oxides. Plutonium metal is soluble in mixtures of nitric acid and hydrofluoric acid (Facer, et al., U.S. Pat. No. 2,942,938), and in nitric acid containing hydrazine and catalytic amounts of fluoride anions (Hopkins, et al., U.S. Pat. No. 3,259,473). Plutonium oxide can also be dissolved in a solution that contains nitric acid, plutonium in solution and fluoride ions in a small, catalytically active amount (Stoll, et al., U.S. Pat. No. 4,434,137). In commonly-assigned U.S. Pat. No. 5,135,728, Karraker describes a method for dissolving delta-phase plutonium in a mixture of nitric acid, hydroxylammonium nitrate (HAN), potassium fluoride and sulfamic acid.
Fluoride-containing mineral acid solutions are also used to recover other actinides. For example, Smith (U.S. Pat. No. 2,741,541) recovers uranium from mixtures containing lower uranium oxides by treating the oxide with dilute aqueous sulfuric acid containing a fluoride. Steahly, et al. (U.S. Pat. No. 2,546,933) dissolve thorium and thorium compounds in nitric acid containing a small amount of fluorine-containing compounds such as hydrogen fluoride, fluosilicic acid, ammonium fluosilicate, and the like.
Sulfamic acid is used in several processes. Jenkins (U.S. Pat. No. 3,208,817) dissolves plutonium metal in a mixture of sulfamic and nitric acids. Sulfamic acid is used as a reductant to adjust plutonium in aqueous nitric acid solution to the Pu.sup.+3 oxidation state (Nemoto, et al., U.S. Pat. No. 4,197,274; Overholt, et al., U.S. Pat. No. 2,863,718). The small amount of sulfamic acid (.ltoreq.0.1M) in the Karraker mixture is not sufficient to dissolve plutonium but assures stability of the hydroxylammonium nitrate in the presence of nitric acid (U.S. Pat. No. 5,135,728).
Typical plutonium dissolution processes focus on producing a product solution that can be fed directly to a solvent extraction process, usually a nitric acid-based process. These dissolution processes use highly corrosive mixtures, such as mixtures of concentrated nitric acid and hydrofluoric acid, that require process equipment made of special halide-resistant alloys. Alternatively, stainless steel vessels must be provided with platinum or corrosion-resistant polytetrafluoroethylene liners that are costly or impractical for use on an industrial scale. Many dissolution processes have unpredictable production rates, side reactions that generate large amounts of hydrogen, and produce unacceptable quantities of plutonium-containing residues including hydrides. Hydrogen must be handled carefully to prevent its concentration from approaching the explosive limit. Production of plutonium-containing residues reduces the yield of the process. In addition, plutonium hydride is pyrophoric, that is, it oxidizes rapidly in air and can ignite spontaneously during servicing of process equipment.
There is a need for a plutonium dissolution process that rapidly and efficiently dissolves plutonium metal, produces a minimal quantity of plutonium-containing waste, and does not require process equipment made of costly, halide-resistant materials.